Magnetic resonance imaging (MRI) is a medical imaging modality that can create images of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a powerful magnet to create a strong, uniform, static magnetic field. When a human body, or part of a human body, is placed in the main magnetic field, the nuclear spins that are associated with the hydrogen nuclei in tissue or fat become polarized. This means that the magnetic moments that are associated with these spins become preferentially aligned along the direction of the main magnetic field, resulting in a small net tissue magnetization along that axis. An MRI system also comprises components called gradient coils that produce smaller amplitude, spatially varying magnetic fields when a current is applied to them. Typically, gradient coils are designed to produce a magnetic field component that is aligned along the z-axis and that varies linearly in amplitude with position along one of the x, y, or z-axes. The effect of a gradient coil is to create a small ramp on the magnetic field strength and, in turn, on the resonant frequency of the nuclear spins along a single axis. Three gradient coils with orthogonal axes are used to “spatially encode” the MRI signal by creating a signature resonance frequency at each location in the body. Typically a radio frequency (RF) body coil is used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei. The RF body coil is used to add energy to the nuclear spins in a controlled fashion. As the nuclear spins then relax back to their rest energy state, they give up energy in the form of an RF signal. The RF signal is detected by one or more RF receive coils and is transformed into an image using a computer and known reconstruction algorithms.
In order to work most effectively, it is important that the RF receive coils are isolated from electrical noise and stray currents. The RF receive coils typically pass the RF signal to a processor in the MRI system by way of one or more coaxial cables. Even though the coaxial cables typically include a layer of conductive shielding, it is possible for currents to be induced on the outer conductive shielding during transmit and receive phases. These induced currents distort the original transmit or receive fields and need to be minimized for optimal imaging. In addition to degrading the image quality, having excessive RF current on the coaxial cables can lead to overheating within the RF receive coils. Since the RF receive coils are typically placed very close to the patient, overly high temperatures can also lead to patient discomfort. A typical technique used to eliminate stray or induced currents on the conductive shielding of the coaxial cables involves creating a high impedance by placing multiple RF traps along the conductive shielding of the coaxial cables.
In a conventional MRI system, each RF trap is typically tuned to a single frequency. For example, in a 3T MRI system, each RF trap is tuned so that it creates a high impedance at the resonance frequency of H (hydrogen), which is around 128 MHz. However, recent developments have shown that a double-tuned RF coil could be useful for creating images at more than one resonant frequency. For example, some of the double-tuned RF coils are used to obtain RF signals from both hydrogen and C13 (carbon 13). In order to eliminate the problems associated with excess RF current on the coaxial cable, it is necessary to have RF traps to eliminate excess current at the resonant frequency of H and at the resonant frequency of C13. For a 3T system, this equates to a resonant frequency of approximately 128 MHz for H and approximately 31 MHz for C13. Using conventional designs, RF traps tuned to 128 MHz and separate RF traps tuned to 31 MHz would be needed for the coaxial cables of a 3T MRI system using a double-tuned RF coil. However, modern MRI systems are very tightly packaged, particularly in the region surrounding the RF coil and associated coaxial cables. It is clear that simply increasing the number of RF traps will lead to wasting unnecessary space. Also, since there is a desire both to keep the patient bore as large as possible for patient comfort and to have the smallest possible magnet to minimize the cost of the MRI system, it is clearly undesirable to add additional space-consuming RF traps to existing designs. Therefore, in order to address these problems as well as others, there is a need for an RE trap that is tuned for multiple resonant frequencies.